The present invention relates to a power converter, which operates with high efficiency and low noise.
Since a switching type power converter has been proposed, the size of a power unit in an electronic apparatus has become smaller. However, due to the rapid progress of semiconductor technology, a power unit is requested to be smaller in size than that at present. Also, a power unit for an electronic apparatus is requested to operate with higher efficiency, and lower noise.
A prior power unit with a single transistor is shown in FIG. 1. In FIG. 1, the reference numeral 1 is a battery or a DC power source for providing the DC voltage E.sub.i, 2 is a switching element implemented by a transistor, 3 is a transformer. The primary winding 3a of the transformer 3 and the switching element 2 are connected in a series, and are coupled with the battery 1. The numeral 3b is a secondary winding of the transformer 3, 4 and 5 are diodes for rectifying the output voltage across the secondary winding 3b. The numeral 6 is a choke coil, and the numeral 7 is a capacitor. The coil 6 and the capacitor 7 operate as an output filter circuit. The numeral 8 is a load coupled across the capacitor 7. The numeral 9 is an error voltage amplifier which monitors the output voltages across the load 8 and provides the error voltage which is the difference between the output voltage across the load and the predetermined reference voltage. The numeral 10 is a control circuit for controlling the switching operation of the transistor 2 (ON and OFF) according to the output of the error voltage amplifier 9. The control circuit 10 adjusts the ON duration, and/or the OFF duration of the transistor 2. The reference numeral 11 is a surge absorption circuit coupled across the primary winding 3a.
The power of the battery 1 is converted to an AC voltage by a switching transistor, and that AC voltage appears across the secondary winding 3b of the transformer 3. The output power of the transformer 3 is applied to the load 8 through the output filter circuit having the choke coil 6 and the capacitor 7. The output voltage across the load 8 is monitored by the error voltage amplifier 9, which controls the ON/OFF operation of the transistor 2 through the control circuit 10 so that the desired output voltage is obtained across the load 8. The surge absorption circuit 11 absorbs the surge voltage induced in the transformer 3 at the OFF transition of the transistor 2 to protect the transistor 2.
FIG. 2 and FIG. 3 show curves of the switching loss in a switching transistor, in which FIG. 2 shows the case of a slow switching operation, and FIG. 3 shows the case of a rapid switching operation. In FIGS. 2 and 3, the symbol V shows the voltage applied to the switching transistor 2, I is the current in the switching transistor 2, and P is the power in the switching transistor 2.
In FIG. 2 where the switching operation is slow, it is supposed that the switching transistor 2 is in the OFF status until the time t.sub.1, said switching transistor 2 is turned to the ON status at the time t.sub.1 and remains in the ON status during the time t.sub.1 and t.sub.3. The duration between the time t.sub.1 and t.sub.2 is the ON transition time. At the time t.sub.3, the transistor 2 is turned to the OFF status, and the time during t.sub.3 and t.sub.4 is the OFF transition time from the ON status to the OFF status.
When the transistor 2 is turned to the ON status, the collector voltage V of the transistor 2 is reduced slowly according to the switching characteristics of the transistor 2 from the initial voltage E.sub.i of the battery voltage, and reaches zero (0) volt at the time t.sub.2, on the condition that the transistor 2 is completely saturated. The current I in the transistor 2 increases slowly according to the switching characteristics of the transistor 2. Therefore, the power P which is the prouct of the voltage V and the current I, and is the loss in the transistor 2 during the ON transition between t.sub.1 and t.sub.2, is rather large. During the time t.sub.2 and the time t.sub.3, the transistor 2 remains in the ON status, the voltage V is zero, and the current I still increases due to the presence of the inductance of the choke coil 6. When the transistor 2 is turned to OFF at the time t.sub.3, the current I reduces slowly according to the characteristics of the transistor 2, and reaches zero at the time t.sub.4. In the OFF transition, a voltage is induced on the leakage inductance of the transformer 3, and then, the voltage V is the sum of the leakage voltage and the voltage E.sub.i of the battery 1, and therefore, the power loss P is considerably larger in the OFF transition between t.sub.3 and t.sub.4. After the time t.sub.4, the current I remains zero, and the voltage V rises up to the clamp voltage E.sub.c because of the flyback voltage of the transformer, and then, the voltage V returns to the source voltage E.sub.i according to the decrease of said flyback voltage.
Next, in case of rapid switching operation as shown in FIG. 3, when the transistor 2 is turned to the ON status, the voltage V of the transistor 2 becomes zero quickly at the time t.sub.2 ', although the current I increases relatively slowly due to the primary leakage inductance of the transformer 3. Therefore, the power loss P in the ON transition between t.sub.1 and t.sub.2 ' is very small. When the transistor 2 is turned to the OFF status at the time t.sub.3, the voltage V increases to the clamp voltage E.sub.c rapidly because of the high leakage voltage which reflects the high decrease of the current I in the transistor 2. At that time, the current I decreases to zero until the time t.sub.4 ' relatively slowly, because the voltage V is restricted to the clamp voltage E.sub.c and the decrease rate of the current in the transistor 2 is small. Therefore, the power loss in the OFF transition between t.sub.3 and t.sub.4 ' is still rather large.
As mentioned above, the power loss P in the switching transistor is consisted mainly of the OFF transition loss, which depends upon the leakage inductance of the transformer 3 and the clamp voltage E.sub.c of the surge absorption circuit 11. Therefore, the power loss P does not decrease even when a high speed switching transistor is used.
The large power loss in a switching transistor results in the generation of heat in an apparatus, and the temperature of an apparatus as a result becomes high. Therefore, the size of the apparatus of FIG. 1 cannot be small enough, because of the high operational temperature and the decrease of the operational reliability due to that high temperature.